1. Field of the Invention
The present invention relates to the field of chemistry and concerns hardenable masses, such as are used, for example, for the production of coatings, and a process for their production and processing.
2. Discussion of Background Information
The blocking of isocyanates by means of dimerization to uretdiones is an effective method of protecting isocyanate groups from premature reaction. The coating of heat-resistant substrates, in particular with powder coatings, is an important area of application for hardeners containing uretdione groups.
Powder coatings are free of solvents and, as a result, are free of harmful emissions to a large extent and they have a very high coating equivalent. Thus, powder coating is a very environmentally friendly and economical coating method.
Polyurethane-based coatings especially distinguish themselves with special light and weather resistance and excellent optical characteristics. The polyurethane powder coatings used today in practice are based on polyesters carrying fixed hydroxyl groups as well as fixed aliphatic and/or cycloaliphatic polyisocyanates whose isocyanate functions are protected by external, low-molecular masking agents. These masking agents escape at least partially during the thermal hardening of the powder coating. These emissions run contrary to the principle of a lack of emissions in powder coating formulations and, for reasons of ecology and workplace hygiene, require special provisions.
By using cross-linkers containing uretdione groups (e.g., DE 23 12 391 OS, EP 045 998 A1, EP 669 353 A1), the possibility exists of avoiding such emissions of low-molecular substances. Due to the low level of reactivity of the internally blocked isocyanate groups, the use of corresponding hardeners containing uretdione groups has been limited up to the present day because the temperatures of greater than 160° C. necessary for hardening are too high and/or the time necessary for hardening is too long. In view of the costs of energy and the possibility of coating thermolabile substrates (e.g., plastic or wood), it is necessary to increase the reactivity of such systems.
Various attempts have been made to accelerate the hardening of polyurethane masses based on hardeners containing uretdione groups by using catalysts. A series of compounds has been suggested, such as the known metalorganic catalysts known from polyurethane chemistry such as tin (II) acetate, tin (II) octoate, tin (II) ethyl caproate, tin (II) laurate, dibutyltin diacetate, dibutyltin dilaureate, dibutyltin maleate (e.g., EP 045 994 A1, EP 045 998 A1, WO 91/07452, or DE 24 20 475), iron (III) chloride, zinc chloride, zinc 2 ethyl caproate, and molybdenum glykolate or tertiary amines such as triethylamine, pyridine, methylpyridine, benzyl dimethylamine, N,N endo ethylenepiperazine, N methylpiperidine, pentamethyldiethylene triamine, N,N dimethylaminocyclohexane, and N,N′ dimethylpiperazine (e.g., n EP 639 598 A1).
Generally, organic tin compounds of the type mentioned are used in practice. The increases in reactivity that can be achieved by means of these catalysts do not yet meet the requirements of masses hardening at low temperatures.
In EP 803 524 A1, compounds containing N,N′ trisubstituted amidines as breakdown catalysts are described. The object of this prior art is a polyurethane powder coating that contains a bonding agent component having hydroxyl groups, a polyaddition compound having uretdione groups and, optionally, free isocyanate groups based on aliphatic and/or cycloaliphatic diisocyanates, at least one N,N,N′ trisubstituted catalyst containing amidine structures with an amidine group content (calculated as CN2; molecular weight=40) of 12.0 to 47.0 wt-%, optionally other catalysts known from polyurethane chemistry, and optionally other auxiliary agents and additives known from powder coating technology. The bonding agent components having hydroxyl groups are added to the powder coating in such amounts that 0.6 to 1.4, preferably 0.8 to 1.2 isocyanategroups of the uretdione groups and optionally polyaddition compounds having free isocyanate groups occur on each hydroxyl group of the bonding agent component, with isocyanate groups being understood as the total of isocyanate groups present in dimeric form and free isocyanate groups. In the publication, possible hardening temperatures of 130° C. and up are mentioned. However, it can be seen in the exemplary embodiments that, at hardening temperatures up to 170° C., coatings of a high degree of quality are not achieved.